The present invention relates to a process for recovering uranium from a wet-process phosphoric acid crude solution containing uranium, by using a solvent extraction process.
Phosphate rock Ca.sub.3 (PO.sub.4).sub.2, which is a raw material for producing phosphoric acid, contains uranium in an amount of about 0.01 wt. % in average, although the amount varies depending upon its locality. Wet-process phosphoric acid is usually produced by the reaction of phosphate rock with sulfuric acid and almost all of the uranium contained in phosphate rock is transferred into the phosphoric acid, hence the resulting crude phosphoric acid contains uranium in an amount of 0.1 g/l in average. The phosphoric acid just after its production contains both tetravalent and hexavalent uranium. In the phosphoric acid produced from uncalcined phosphate rock, the ratio of U.sup.4+ to U.sup.6+ is approximately 1:1, while in the phosphoric acid produced from calcined phosphate rock, the most part of the resulting uranium in U.sup.4+. In our country, phosphoric acid has been produced mainly from uncalcined phosphate rock. Anyway, U.sup.4+ is gradually oxidized with the passage of time, and all of the uranium turns to U.sup.6+ after all. However, in the case where uranium is recovered as a by product from wet-process phosphoric acid, since the uranium recovering step is incorporated into the steps of phosphoric acid production or phosphate fertilizer production, the phosphoric acid to be sent to the uranium-recovering step is not so long in the elapse of time after its production, and contains tetravalent and hexavalent uranium in admixture, its temperature being about 40.degree. C.
As for conventional processes for recovering uranium from wet-process phosphoric acid containing tetravalent and hexavalent uranium in admixture as mentioned above, there are an oxidative stripping process as disclosed in U.S. Pat. No. 3,835,214 and a reductive stripping process as disclosed in U.S. Pat. No. 3,711,591. In these prior art processes, uranium has been extracted with a solvent after reduction or oxidation of phosphoric acid. Namely, in the case of the former, uranium in the phosphoric acid is reduced to turn all of the uranium tetravalent, and then extract with octylphenyl phosphoric acid (hereinafter abbreviated to OPPA), and in the subsequent back-extraction, the uranium is oxidized into hexavalent one so as to be easily back-extracted. In the case of the latter, uranium in the phosphoric acid is oxidized to turn all of the uranium hexavalent, and then the hexavalent uranium is extracted with an extractant that extracts hexavalent uranium alone, i.e. with an organic extractant consisting of di(2-ethylhexyl)phosphoric acid (hereinafter abbreviated to D2EHPA) and trioctylphosphine oxide (hereinafter abbreviated to TOPO) dissolved in a diluent, and in the subsequent back-extraction, the uranium is reduced into tetravalent one so as to be easily back-extracted. According to such conventional processes, since metallic iron is mainly employed in the reduction, the amount of iron as an impurity in the phosphoric acid product increases, while since sodium chlorate is mainly employed in the oxidation, problems of corrosion of apparatus and occurrence of by-produced gas are raised, and besides there are drawbacks such that the recovery step, the recovery equipments, etc., are complicated.